weblayer: adds calls to WindowAndroid.destroy

BUG=1018148
TEST=none

Change-Id: Icb07328be04799cd4d7c87d4f07b47976aa33982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904501Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713641}

weblayer/browser/java/org/chromium/weblayer_private/BrowserImpl.java

@@ -184,7 +184,9 @@ public class BrowserImpl extends IBrowser.Stub {
             }
             mViewController = null;
         }
-        mWindowAndroid = null;
-        mViewController = null;
+        if (mWindowAndroid != null) {
+            mWindowAndroid.destroy();
+            mWindowAndroid = null;
+        }
     }
 }